[MacCatalyst] DatePicker null date handling

[jsuarezruiz]

Description of Change

Created an UITest to validate the behavior and seems to be the same on iOS MacCatalyst. When the DatePicker Date value is set to null and the date picker is opened, the picker keeps today date.

Expected behavior

• When DatePicker.Date is null, the MacCatalyst picker will display today's date when opened
• The virtual view Date property remains null (as expected)
• Transitions between null and specific dates work correctly
• Existing functionality with non-null dates remains unchanged

Issues Fixed

Fixes #31124

[Copilot]

Pull Request Overview

This PR adds a UI test to validate DatePicker behavior when its Date property is set to null on macOS platforms. The test ensures that when a DatePicker's Date is set to null, the picker maintains today's date when opened while the virtual view Date property correctly remains null.

• Adds comprehensive UI test infrastructure for Issue 31124
• Creates test page with DatePicker, button, and label components for validation
• Implements automated test to verify null date handling behavior

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Controls/tests/TestCases.HostApp/Issues/Issue31124.cs	Creates UI test page with DatePicker, button to set null date, and label to display current date
src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue31124.cs	Implements NUnit test to validate DatePicker behavior when date is set to null

[Copilot]

This line will throw a NullReferenceException when _datePicker.Date is null. The format specifier :d cannot be applied to a null DateTime?. Consider using conditional formatting like _datePicker.Date?.ToString("d") ?? "(null)" or similar null-safe approach.

Suggested change

	_dateLabel.Text = $"Current Date: {_datePicker.Date:d}";
	_dateLabel.Text = $"Current Date: {_datePicker.Date?.ToString("d") ?? "(null)"}";

[Copilot]

The System.Text.RegularExpressions namespace is imported but never used in this file. Consider removing this unused import.

Suggested change

using System.Text.RegularExpressions;